A fully active, two-active-site, single-chain sucrase.isomaltase from pig small intestine. Implications for the biosynthesis of a mammalian integral stalked membrane protein

Probing paracellular -versus transcellular tissue barrier permeability using a gut mucosal explant culture system

Intestinal permeation enhancers (PEs), i.e. agents improving oral delivery of therapeutic drugs with poor bioavailability, may typically act by two principally different mechanisms: to increase either transcellular -or paracellular passage across the epithelium. With the aim to define these different modes of action in a small intestinal mucosal explant system, the transcellular-acting PE sodium dodecyl sulfate (SDS) was compared to the paracellular-acting PE ethylenediaminetetraacetic acid (EDTA), using several fluorescent polar - and lipophilic probes. Here, SDS rendered the enterocyte cell membranes leaky for the relatively small polar tracers Lucifer yellow and a 3 kD Texas red-conjugated dextran, but most conspicuously SDS blocked constitutive endocytosis from the brush border. In contrast, the main action of EDTA was to increase paracellular passage across the epithelium of both polar probes, including 10 - and 70 kDa dextrans and lipophilic probes, visualized by distinct stripy lateral staining of enterocytes and/or accumulation in the lamina propria. In addition, EDTA caused a loss of epithelial cell polarity by opening tight junctions for diffusion of domain-specific basolateral/apical cell membrane protein markers into the opposite domains. By transmission electron microscopy, SDS caused the formation of vacuoles and vesicle-like structures at the lateral cell membranes. In contrast, EDTA led to a bulging of the whole enterocyte apex, resulting in a "cobblestone" appearance of the epithelium, probably caused by an extreme contraction of the perijunctional actomyosin ring. We conclude that the mucosal explant system is a convenient model for predicting transcellular/paracellular modes of action of novel prospective PEs.

Determination Trial of Nondigestible Oligosaccharide in Processed Foods by Improved AOAC Method 2009.01 Using Porcine Small Intestinal Enzyme

We have previously shown that the Association of Official Analytical Chemists' (AOAC) methods 2001.03 and 2009.01 were not able to measure accurately nondigestible oligosaccharide because they are incapable of hydrolyzing digestible oligosaccharide, leading to overestimation of nondigestible oligosaccharide. Subsequently, we have proposed improved AOAC methods 2001.03 and 2009.01 using porcine small intestinal disaccharidases instead of amyloglucosidase. In the present study, we tried to determine nondigestible oligosaccharide in marketed processed foods using the improved AOAC method (improved method), and the results were compared with those by AOAC method 2009.01. In the improved method, the percentages of recovery of fructooligosaccharide, galactooligosaccharide, and raffinose to the label of processed food were 103.0, 89.9, and 102.1%, respectively. However, the AOAC method 2009.01 overestimated >30% of the quantity of nondigestible oligosaccharide in processed foods, because the margin of error was accepted ±20% on the contents of nondigestible oligosaccharides in processed foods for Japanese nutrition labeling, the improved method thus provided accurate quantification of nondigestible oligosaccharides in processed food and allows a comprehensive determination of nondigestible oligosaccharides.

The Secretion and Action of Brush Border Enzymes in the Mammalian Small Intestine

Microvilli are conventionally regarded as an extension of the small intestinal absorptive surface, but they are also, as latterly discovered, a launching pad for brush border digestive enzymes. Recent work has demonstrated that motor elements of the microvillus cytoskeleton operate to displace the apical membrane toward the apex of the microvillus, where it vesiculates and is shed into the periapical space. Catalytically active brush border digestive enzymes remain incorporated within the membranes of these vesicles, which shifts the site of BB digestion from the surface of the enterocyte to the periapical space. This process enables nutrient hydrolysis to occur adjacent to the membrane in a pre-absorptive step. The characterization of BB digestive enzymes is influenced by the way in which these enzymes are anchored to the apical membranes of microvilli, their subsequent shedding in membrane vesicles, and their differing susceptibilities to cleavage from the component membranes. In addition, the presence of active intracellular components of these enzymes complicates their quantitative assay and the elucidation of their dynamics. This review summarizes the ontogeny and regulation of BB digestive enzymes and what is known of their kinetics and their action in the peripheral and axial regions of the small intestinal lumen.

Functional regulation of sugar assimilation by N-glycan-specific interaction of pancreatic α-amylase with glycoproteins of duodenal brush border membrane

Porcine pancreatic α-amylase (PPA) binds to N-linked glycans of glycoproteins (Matsushita, H., Takenaka, M., and Ogawa, H. (2002) J. Biol Chem., 277, 4680-4686). Immunostaining revealed that PPA is located at the brush-border membrane (BBM) of enterocytes in the duodenum and that the binding is inhibited by mannan but not galactan, indicating that PPA binds carbohydrate-specifically to BBM. The ligands for PPA in BBM were identified as glycoprotein N-glycans that are significantly involved in the assimilation of glucose, including sucrase-isomaltase (SI) and Na(+)/Glc cotransporter 1 (SGLT1). Binding of SI and SGLT1 in BBM to PPA was dose-dependent and inhibited by mannan. Using BBM vesicles, we found functional changes in PPA and its ligands in BBM due to the N-glycan-specific interaction. The starch-degrading activity of PPA and maltose-degrading activity of SI were enhanced to 240 and 175%, respectively, while Glc uptake by SGLT1 was markedly inhibited by PPA at high but physiologically possible concentrations, and the binding was attenuated by the addition of mannose-specific lectins, especially from Galanthus nivalis. Additionally, recombinant human pancreatic α-amylases expressed in yeast and purified by single-step affinity chromatography exhibited the same carbohydrate binding specificity as PPA in binding assays with sugar-biotinyl polymer probes. The results indicate that mammalian pancreatic α-amylases share a common carbohydrate binding activity and specifically bind to the intestinal BBM. Interaction with N-glycans in the BBM activated PPA and SI to produce much Glc on the one hand and to inhibit Glc absorption by enterocytes via SGLT1 in order to prevent a rapid increase in blood sugar on the other.

Dietary cholesterol induces trafficking of intestinal Niemann-Pick Type C1 Like 1 from the brush border to endosomes

The transmembrane protein Niemann-Pick C1 Like 1 (NPC1L1) belongs to the Niemann-Pick C1 (NPC1) family of cholesterol transporters and is mainly expressed in the liver and the small intestine. NPC1L1 is believed to be the main transporter responsible for the absorption of dietary cholesterol. Like NPC1, NPC1L1 contains a sterol sensing domain, suggesting that it might be sensitive to dietary cholesterol. To test this hypothesis, mucosal explants were cultured in the presence or absence of cholesterol. In the absence of cholesterol NPC1L1 was localized mainly in the brush border of the enterocyte, colocalizing with the brush border enzyme aminopeptidase N (APN), and only a minor part was present in intracellular compartments. In contrast, following culture in the presence of cholesterol a major part of NPC1L1 was found in intracellular compartments positive for the early endosomal marker early endosome antigen 1, whereas only a minor fraction was left in the brush border. Neither APN, lactase, nor sucrase-isomaltase was endocytosed in parallel, demonstrating that this is a selective cholesterol-induced endocytosis of NPC1L1. Conceivably either the induced internalization could be due to NPC1L1 acting as an endocytic cholesterol receptor or it could be a mechanism to reduce the cholesterol uptake. The fluorescent cholesterol analog NBD-cholesterol readily labeled the cytoplasm also under conditions nonpermissible for endocytosis, arguing against a receptor-mediated uptake. We therefore propose that cholesterol is absorbed by NPC1L1 acting as a membrane transporter and that NPC1L1 is internalized to an endosomal compartment to reduce the absorption of cholesterol.

Galectin-2 at the enterocyte brush border of the small intestine

The brush border of pig small intestine is a local hotspot for beta-galactoside-recognizing lectins, as evidenced by its prominent labeling with fluorescent lectin PNA. Previously, galectins 3-4, intelectin, and lectin-like anti-glycosyl antibodies have been localized to this important body boundary. Together with the membrane glycolipids these lectins form stable lipid raft microdomains that also harbour several of the major digestive microvillar enzymes. In the present work, we identified a lactose-sensitive 14-kDa protein enriched in a microvillar detergent resistant fraction as galectin-2. Its release from closed, right-side-out microvillar membrane vesicles shows that at least some of the galectin-2 resides at the lumenal surface of the brush border, indicating that it plays a role in the organization/stabilization of the lipid raft domains. Galectin-2 was released more effectively from the membrane by lactose than was galectin-4, and surprisingly, it was also released by the noncanonical disaccharides sucrose and maltose. Furthermore, unlike galectin-4, galectin-2 was preferentially co-immunoisolated with sucrase-isomaltase rather than with aminopeptidase N. Together, these results show that the galectins are not simply redundant proteins competing for the same ligands but rather act in concert to ensure an optimal cross-linking of membrane glycolipids and glycoproteins. In this way, they offer a maximal protection of the brush border against exposure to bile, pancreatic enzymes and pathogens.

Biosynthesis and assembly of the largest and major intrinsic polypeptide of the small intestinal brush borders

The sucrase-isomaltase complex (SI) of the small intestinal brush border membrane accounts for approximately 9-10% of the intrinsic protein. The isomaltase subunit alone interacts with the membrane directly, via a highly hydrophobic segment at its N-terminal region. This segment has a helical conformation for more than 85% and crosses the membrane twice, the N-terminus being located at the outer, luminal side of the membrane. The sucrase subunit is attached to the membrane solely via its interactions with the isomaltase subunit. The sucrase-isomaltase complex is synthesized as a single, very long (Mr approximately 260 000) polypeptide chain (pro-SI, carrying the two sites of sucrase and isomaltase in an already enzymically active form), with the isomaltase portion corresponding to the N-terminal part of pro-SI. Pro-SI is processed into 'final' SI by pancreatic proteases. Recently the cell-free translation of pro-SI has been achieved in vitro. From a detailed knowledge of the anchoring of SI (and pro-SI) in the membrane it has been possible to suggest one particular mechanism as the most likely for the synthesis, insertion and assembly of pro-SI.

Use of monoclonal antibodies in the study of intestinal structure and function

The hybridoma technique, originally developed by G. Kohler & C. Milstein, is a powerful new experimental approach for analysis of complex biological systems, and is particularly suited for identification and study of surface-membrane antigens. This technique has been used for the production of monoclonal antibodies to intestinal brush border membrane proteins. Spleen cells, obtained from BALB/c mice immunized with purified brush border membranes, were fused with NSI mouse myeloma cells, and hybrids were selected with a culture medium containing hypoxanthine, aminopterin and thymidine (HAT medium). Hybridoma cultures were screened for production of specific antibodies by radio-immunobinding assays and by immunofluorescent staining of intestinal frozen sections. Selected hybridoma cultures were cloned twice and used for the production of large amounts of antibodies, which were characterized. Nineteen monoclonal antibodies have been prepared to date, about half of them specifically staining the brush border membrane of mature enterocytes. Ten of the antibodies specifically immunoprecipitate surface-membrane proteins, which were analysed by sodium dodecyl sulphate slab-gel electrophoresis, by two-dimensional slab-gel electrophoresis, and by specific enzyme assays. Two antibodies were found to be specific for sucrase-isomaltase, one for an aminopeptidase, two for an isoenzyme of alkaline phosphatase that is present exclusively in the proximal small intestine, and one for maltase-glucoamylase. These monoclonal antibodies, and others prepared by similar techniques from mice immunized with a wide variety of intestinal subcellular fractions, should prove invaluable tools for the study of the biosynthesis of cell-surface proteins, the fetal and postnatal development of specific intestinal functions, and the process of cell differentiation in the intestinal epithelium.

Biosynthesis and transport of plasma membrane glycoproteins in the rat intestinal epithelial cell: studies with sucrase-isomaltase

Sucrase-isomaltase (SI), an integral heterodimeric glycoprotein of the intestinal microvillus membrane, is synthesized as a single enzymically active precursor protein (pro-SI) of high relative molecular mass. After glycosylation in the Golgi complex pro-SI is transferred to the microvillus membrane where it is cleaved into the two subunits by pancreatic elastase. Pro-SI was purified by monoclonal antibody-affinity chromatography from microvillus membranes of fetal intestinal transplants in which SI is found exclusively in the non-cleaved precursor form. The N-terminal amino acid sequence of pro-SI was identical to that of the isomaltase subunit of SI which anchors the mature enzyme complex to the lipid bilayer, but it differed from the N-terminal sequence of the sucrase subunit of SI. This structural comparison indirectly gave insight into the mechanisms of membrane insertion and assembly of pro-SI during its biosynthesis. Subcellular fractionation studies indicate transient structural association of newly synthesized pro-SI with the basolateral membrane on its transfer from the Golgi complex to the microvillus membrane, suggesting that part of the basolateral membrane or its associated structures might be involved in the sorting-out processes of microvillar membrane proteins. This concept may have general relevance for the mechanisms of membrane insertion, intracellular transport and sorting of other microvillar membrane glycoproteins in the intestinal epithelial cell.

Structure of microvillar enzymes in different phases of their life cycles

Structural changes have been studied during the life cycles of three glycosidases: sucrase-isomaltase (EC 3.2.48-10), lactase-phlorizin hydrolase (EC 3.2.1.23-62), maltase-glucoamylase (EC 3.2.1.20); and three peptidases: aminopeptidase A (EC 3.4.11.7), aminopeptidase N (EC 3.4.11.2) and dipeptidyl peptidase IV (EC 3.4.14.5). The final forms of the enzymes can be divided into at least two groups: the sucrase-isomaltase type, characterized as dimers, which are asymmetric in their hydrophilic parts, have two types of active site and anchor only on one subunit; and the aminopeptidase N type, characterized as dimers, which are symmetric in their hydrophilic part, have only one type of active site and anchor on both subunits. These enzymes are likely to be synthesized on rough endoplasmic reticulum and simultaneously glycosylated into endoglycosidase H-sensitive forms. They are later reglycosylated to endoglycosidase H-resistant forms, which have relative molecular masses similar to the final forms. Enzymes of the sucrase-isomaltase type seem to be synthesized with a polypeptide-chain length corresponding to the sum of both subunits, whereas enzymes of the aminopeptidase N type seem to be synthesized with a polypeptide-chain length corresponding to the constituent subunits themselves. Not much is known about the catabolism of these enzymes. The enzyme activities and the amounts of enzyme protein decrease at the top of the villi, probably due to release into the lumen. The subunits of aminopeptidase N are cleaved by pancreatic proteases to smaller peptides, and sucrase-isomaltase may lose its sucrase polypeptide, while both enzymes remain bound to the membrane.

Anti-glycosyl antibodies in lipid rafts of the enterocyte brush border: a possible host defense against pathogens

The pig small intestinal brush border is a glycoprotein- and glycolipid-rich membrane that functions as a digestive/absorptive surface for dietary nutrients as well as a permeability barrier for pathogens. The present work was performed to identify carbohydrate-binding (lectinlike) proteins associated with the brush border. Chromatography on lactose-agarose was used to isolate such proteins, and their localization was studied biochemically and by immunofluorescence microscopy and immunogold electron microscopy. IgG and IgM were the two major proteins isolated, indicating that naturally occurring anti-glycosyl antibodies are among the major lectinlike proteins in the gut. IgG and IgM as well as IgA were localized to the enterocyte brush border, and a brief lactose wash partially released all three immunoglobulins from the membrane, indicating that anti-glycosyl antibodies constitute a major part of the immunoglobulins at the lumenal surface of the gut. The antibodies were associated with lipid rafts at the brush border, and they frequently (52%) coclustered with the raft marker galectin 4. A lactose wash increased the susceptibility of the brush border toward lectin peanut agglutin and cholera toxin B, suggesting that anti-glycosyl antibodies compete with other carbohydrate-binding proteins at the lumenal surface of the gut. Thus anti-glycosyl antibodies constitute a major group of proteins associated with the enterocyte brush border membrane. We propose they function by protecting the lipid raft microdomains of the brush border against pathogens.

Cholera toxin entry into pig enterocytes occurs via a lipid raft- and clathrin-dependent mechanism

The small intestinal brush border is composed of lipid raft microdomains, but little is known about their role in the mechanism whereby cholera toxin gains entry into the enterocyte. The present work characterized the binding of cholera toxin B subunit (CTB) to the brush border and its internalization. CTB binding and endocytosis were performed in organ-cultured pig mucosal explants and studied by fluorescence microscopy, immunogold electron microscopy, and biochemical fractionation. By fluorescence microscopy CTB, bound to the microvillar membrane at 4 degrees C, was rapidly internalized after the temperature was raised to 37 degrees C. By immunogold electron microscopy CTB was seen within 5 min at 37 degrees C to induce the formation of numerous clathrin-coated pits and vesicles between adjacent microvilli and to appear in an endosomal subapical compartment. A marked shortening of the microvilli accompanied the toxin internalization whereas no formation of caveolae was observed. CTB was strongly associated with the buoyant, detergent-insoluble fraction of microvillar membranes. Neither CTB's raft association nor uptake via clathrin-coated pits was affected by methyl-beta-cyclodextrin, indicating that membrane cholesterol is not required for toxin binding and entry. The ganglioside GM(1) is known as the receptor for CTB, but surprisingly the toxin also bound to sucrase-isomaltase and coclustered with this glycosidase in apical membrane pits. CTB binds to lipid rafts of the brush border and is internalized by a cholesterol-independent but clathrin-dependent endocytosis. In addition to GM(1), sucrase-isomaltase may act as a receptor for CTB.

“Nonclassical” Secretion of Annexin A2 to the Lumenal Side of the Enterocyte Brush Border Membrane†

Annexin A2 is a member of the annexin family of Ca(2+)-dependent lipid binding proteins and believed to be engaged in membrane transport processes in a number of cell types. In small intestinal enterocytes, we localized annexin A2 to the brush border region, where it was found mainly on the lumenal side of the microvilli, showing an apical secretion by a "nonclassical" mechanism. In addition, annexin A2 was associated with surface-connected, deep apical tubules in the apical terminal web region and with an underlying pleiomorphic, tubulo-vesicular compartment (subapical compartment/multivesicular bodies). By subcellular fractionation, the 36 kDa full-length form of annexin A2 was approximately equally distributed between the Mg(2+)-precipitated fraction (containing intracellular and basolateral membranes) and the microvillar membrane fraction. In addition, a 33 kDa molecular form of annexin A2 was seen in the latter fraction that could be generated from the full-length annexin A2 by digestion with trypsin. Taken together, the results suggest that annexin A2 acts in exocytic apical membrane trafficking and is proteolytically cleaved in situ by pancreatic proteinases once it has become externalized to the lumenal side of the brush border membrane. On the basis of its well-known membrane fusogenic properties, we propose a model for the nonclassical membrane translocation of annexin A2.

Yeast glyoxalase I is a monomeric enzyme with two active sites

The tertiary structure of the monomeric yeast glyoxalase I has been modeled based on the crystal structure of the dimeric human glyoxalase I and a sequence alignment of the two enzymes. The model suggests that yeast glyoxalase I has two active sites contained in a single polypeptide. To investigate this, a recombinant expression clone of yeast glyoxalase I was constructed for overproduction of the enzyme in Escherichia coli. Each putative active site was inactivated by site-directed mutagenesis. According to the alignment, glutamate 163 and glutamate 318 in yeast glyoxalase I correspond to glutamate 172 in human glyoxalase I, a Zn(II) ligand and proposed general base in the catalytic mechanism. The residues were each replaced by glutamine and a double mutant containing both mutations was also constructed. Steady-state kinetics and metal analyses of the recombinant enzymes corroborate that yeast glyoxalase I has two functional active sites. The activities of the catalytic sites seem to be somewhat different. The metal ions bound in the active sites are probably one Fe(II) and one Zn(II), but Mn(II) may replace Zn(II). Yeast glyoxalase I appears to be one of the few enzymes that are present as a single polypeptide with two active sites that catalyze the same reaction.

Cholesterol depletion of enterocytes. Effect on the Golgi complex and apical membrane trafficking

Intestinal brush border enzymes, including aminopeptidase N and sucrase-isomaltase, are associated with "rafts" (membrane microdomains rich in cholesterol and sphingoglycolipids). To assess the functional role of rafts in the present work, we studied the effect of cholesterol depletion on apical membrane trafficking in enterocytes. Cultured mucosal explants of pig small intestine were treated for 2 h with the cholesterol sequestering agent methyl-beta-cyclodextrin and lovastatin, an inhibitor of hydroxymethylglutaryl-coenzyme A reductase. The treatment reduced the cholesterol content >50%. Morphologically, the Golgi complex/trans-Golgi network was partially transformed into numerous 100-200 nm vesicles. By immunogold electron microscopy, aminopeptidase N was localized in these Golgi-derived vesicles as well as at the basolateral cell surface, indicating a partial missorting. Biochemically, the rates of the Golgi-associated complex glycosylation and association with rafts of newly synthesized aminopeptidase N were reduced, and less of the enzyme had reached the brush border membrane after 2 h of labeling. In contrast, the basolateral Na(+)/K(+)-ATPase was neither missorted nor raft-associated. Our results implicate the Golgi complex/trans-Golgi network in raft formation and suggest a close relationship between this event and apical membrane trafficking.

Growth, development and differentiation: a functional food science approach

Few other aspects of food supply and metabolism are of greater biological importance than the feeding of mothers during pregnancy and lactation, and of their infants and young children. Nutritional factors during early development not only have short-term effects on growth, body composition and body functions but also exert long-term effects on health, disease and mortality risks in adulthood, as well as development of neural functions and behaviour, a phenomenon called 'metabolic programming'. The interaction of nutrients and gene expression may form the basis of many of these programming effects and needs to be investigated in more detail. The relation between availability of food ingredients and cell and tissue differentiation and its possible uses for promoting health and development requires further exploration. The course of pregnancy, childbirth and lactation as well as human milk composition and the short- and long-term outcome of the child are influenced by the intake of foods and particularly micronutrients, e.g. polyunsaturated fatty acids, Fe, Zn and I. Folic acid supplementation from before conception through the first weeks of pregnancy can markedly reduce the occurrence of severe embryonic malformations; other potential benefits of modulating nutrient supply on maternal and child health should be further evaluated. The evaluation of dietary effects on child growth requires epidemiological and field studies as well as evaluation of specific cell and tissue growth. Novel substrates, growth factors and conditionally essential nutrients (e.g. growth factors, amino acids, polyunsaturated fatty acids) may be potentially useful as ingredients in functional foods and need to be assessed carefully. Intestinal growth, maturation, and adaptation as well as long-term function may be influenced by food ingredients such as oligosaccharides, gangliosides, high-molecular-mass glycoproteins, bile salt-activated lipase, pre- and probiotics. There are indications for some beneficial effects of functional foods on the developing immune response, for example induced by antioxidant vitamins, trace elements, fatty acids, arginine, nucleotides, and altered antigen contents in infant foods. Peak bone mass at the end of adolescence can be increased by dietary means, which is expected to be of long-term importance for the prevention of osteoporosis at older ages. Future studies should be directed to the combined effects of Ca and other constituents of growing bone, such as P, Mg and Zn, as well as vitamins D and K, and the trace elements F and B. Pregnancy and the first postnatal months are critical time periods for the growth and development of the human nervous system, processes for which adequate substrate supplies are essential. Early diet seems to have long-term effects on sensory and cognitive abilities as well as behaviour. The potential beneficial effects of a balanced supply of nutrients such as I, Fe, Zn and polyunsaturated fatty acids should be further evaluated. Possible long-term effects of early exposure to tastes and flavours on later food choice preferences may have a major impact on public health and need to be further elucidated. The use of biotechnology and recombinant techniques may offer the opportunity to include various bioactive substances in special dietary products, such as human milk proteins, peptides, growth factors, which may have beneficial physiological effects, particularly in infancy and early childhood.

Molecular cloning of sucrase-isomaltase cDNA in the house musk shrew Suncus murinus and identification of a mutation responsible for isolated sucrase deficiency

Isolated sucrase deficiency has been demonstrated in a line of house musk shrew, Suncus murinus (laboratory name: suncus). This animal belongs to the order Insectivore and is phylogenetically different from ordinarily used laboratory animals. They are believed to have evolved with mainly animal food without sucrose. To study the molecular basis of the sucrase deficiency in suncus, we cloned 6. 0-kilobase (kb) sucrase-isomaltase (SI, EC 3.2.1.48-10) cDNA from suncus intestinal cDNA library. The cDNA clone contained a 5442-base pair (bp)-long open reading frame preceded by an in frame termination codon. The deduced 1813-amino acid sequence showed 68.6, 71.2, and 74.7% similarity with those of rat, rabbit, and human, respectively. A cleavage site between isomaltase and sucrase as well as the region surrounding the catalytic sites for sucrase and isomaltase were conserved among the species. Out of 18 potential N-linked glycosylation sites, 5 were common among all 4 species. In the connecting segment which was enriched with O-linked glycosylation sites in the other species, only two sites were present in suncus. Northern blot analysis revealed that the 6.0-kb SI mRNA was expressed in the KAT line with intact sucrase-isomaltase activity. In contrast, 3.0-kb SI mRNA was expressed in suncus of the MI line with isolated sucrase deficiency. The 3.0-kb mRNA cosegregated with sucrase deficiency phenotype as an autosomal recessive trait. Sequence analysis revealed a 2-nucleotide deletion at position 2767-2768, which results in a frameshift and an immature termination codon. The cDNA of the MI line diverged from that of the KAT line at position 2865, having an 18-bp unique sequence followed by a poly(A) tail. The mutant cDNA encodes 922 amino acid residues which preserves the region for isomaltase but lacks that for whole sucrase. While the cells transfected with the plasmids expressing SI in the KAT line showed both sucrase and isomaltase activity, the plasmids expressing MI line cDNA showed only isomaltase activity. Thus it was concluded that the mutation in the SI gene was responsible for isolated sucrase deficiency in the MI line.

Forensic application of organ-specific antigens

A highly sensitive sandwich enzyme immunoassay for organ-specific antigen is described for use in forensic practice. The sandwich enzyme immunoassays for specific antigens to the liver (LSA), the small intestine (sucrase-isomaltase), and the heart (cardiac troponin I) were developed. High levels of antigen could be detected to exist in forensic materials, and it is clearly possible to differentiate between samples from these stabbed organs and those originating from other stabbed organs. In addition, a sandwich enzyme immunoassay for prostate-specific antigen (gamma-seminoprotein, gamma-sm) was developed for sex discrimination of blood and bloodstains. The ratio of gamma-sm to hemoglobin was significantly higher in male adults than in female adults.

Disaccharide digestion and maldigestion

All food carbohydrates are hydrolysed to monosaccharides before transport across the microvillus membrane. The digestion of disaccharides and some oligosaccharides is undertaken by a number of small intestinal brush border enzymes: sucrase-isomaltase, lactase phlorizinhydrolase, maltase-glycoamylase and trehalase. The distribution of the enzymes in the small intestine has been investigated. Different disaccharide maldigestion syndromes have been described. Lactase deficiency in adults is a condition found in the majority of inhabitants of the world. However, the prevalence varies widely between different populations. Sucrase-isomaltase deficiency is a very rare congenital condition except in Greenland. Trehalose maldigestion is likewise rare outside Greenland. Different hypotheses regarding the molecular background of the maldigestion syndromes are discussed.

Phosphorylation of the N-terminal intracellular tail of sucrase-isomaltase by cAMP-dependent protein kinase

This paper reports the phosphorylation of the intracellular N-terminal tail of sucrase-isomaltase by protein kinase A and shows that this phosphorylation is targeted to Ser6 within a sequence Arg/Lys/Lys-Phe-Ser, which is conserved in all sucrase-isomaltase sequences known so far. By dephosphorylation of native sucrase-isomaltase with an immobilized acid phosphatase and rephosphorylation with protein kinase A, it is shown that Ser6 may be partially phosphorylated in vivo, raising the possibility that the tail itself and its phosphorylation by protein kinase A may be physiologically significant.

Characterization of microvillar membrane proteins of dog small intestine by two-dimensional electrophoresis

A method for analysing microgram amounts of microvillar membranes by two-dimensional electrophoresis (protein mapping) is described, and has been used to characterize the microvillar proteins of the small intestine of German shepherd, corgi, and beagle dogs. Detergent-solubilized microvillar membranes were radiolabelled with 14C and separated by isoelectric focussing followed by SDS-PAGE. Proteins were detected fluorographically and glycoproteins by lectin-affinity staining. The microvillar hydrolases alkaline phosphatase and dipeptidyl aminopeptidase IV were identified by active-site labelling and aminopeptidase N by immunoprecipitation. Changes following pancreatic duct diversion were consistent with accumulation of pro-sucrase-isomaltase and diminished expression of the sucrase and isomaltase subunits. Cytoskeletal proteins were concentrated in the core fraction remaining after extraction of microvillar membranes with Triton X-100. There were no consistent differences between dogs of different breed, and the canine protein maps were similar to the human.

Intestinal brush border glycohydrolases: structure, function, and development

The hydrolytic enzymes of the intestinal brush border membrane are essential for the degradation of nutrients to absorbable units. Particularly, the brush border glycohydrolases are responsible for the degradation of di- and oligosaccharides into monosaccharides, and are thus crucial for the energy-intake of humans and other mammals. This review will critically discuss all that is known in the literature about intestinal brush border glycohydrolases. First, we will assess the importance of these enzymes in degradation of dietary carbohydrates. Then, we will closely examine the relevant features of the intestinal epithelium which harbors these glycohydrolases. Each of the glycohydrolytic brush border enzymes will be reviewed with respect to structure, biosynthesis, substrate specificity, hydrolytic mechanism, gene regulation and developmental expression. Finally, intestinal disorders will be discussed that affect the expression of the brush border glycohydrolases. The clinical consequences of these enzyme deficiency disorders will be discussed. Concomitantly, these disorders may provide us with important details regarding the functions and gene expression of these enzymes under specific (pathogenic) circumstances.

Induction of lactase biosynthesis in the human intestinal epithelial cell line Caco-2

The human colonic adenocarcinoma cell line Caco-2 forms monolayers of differentiated enterocyte-like cells when cultured on permeable supports. After confluency, Caco-2 cells express a number of brush-border enzymes including lactase-phlorizin hydrolase, sucrase-isomaltase and dipeptidylpeptidase IV. We have studied, with particular emphasis on lactase-phlorizin hydrolase, the modulation of biosynthesis of these enzymes by stimulating second messenger systems. Forskolin induced lactase-phlorizin hydrolase synthesis approximately fourfold within 7 h, suppressed sucrase-isomaltase synthesis, and had little effect on dipeptidylpeptidase IV. Dibutyryl-cAMP, 8-bromo-cAMP and vasoactive intestinal peptide also increased lactase-phlorizin hydrolase biosynthesis, indicating c-AMP dependent regulation. The induction of lactase-phlorizin hydrolase biosynthesis could be inhibited by actinomycin D and was preceded by a fourfold increase in lactase-phlorizin hydrolase mRNA levels, suggesting transcriptional control. Phorbol 12-myristate 13-acetate had an inhibitory effect on brush-border enzyme synthesis, in particular on sucrase-isomaltase, and blocked the forskolin-induced biosynthesis of lactase-phlorizin hydrolase. Lactase-phlorizin hydrolase synthesis was also inducible by hydrocortisone, but maximal induction required at least 3 days during which time sucrase-isomaltase synthesis diminished. The results indicate opposite regulation of lactase-phlorizin hydrolase and sucrase-isomaltase via cAMP and corticosteroids, and suggest that the Caco-2 cell line can serve as a model system to study aspects of the humoral regulation of human intestinal brush-border enzymes in cell culture.

Primary structure and processing of the Candida tsukubaensis alpha-glucosidase. Homology with the rabbit intestinal sucrase-isomaltase complex and human lysosomal alpha-glucosidase

The nucleotide sequence of a 4.39-kb DNA fragment encoding the alpha-glucosidase gene of Candida tsukubaensis is reported. The cloned gene contains a major open reading frame (ORF 1) which encodes the alpha-glucosidase as a single precursor polypeptide of 1070 amino acids with a predicted molecular mass of 119 kDa. N-terminal amino acid sequence analysis of the individual subunits of the purified enzyme, expressed in the recombinant host Saccharomyces cerevisiae, confirmed that the alpha-glucosidase precursor is proteolytically processed by removal of an N-terminal signal peptide to yield the two peptide subunits 1 and 2, of molecular masses 63-65 kDa and 50-52 kDa, respectively. Both subunits are secreted by the heterologous host S. cerevisiae in a glycosylated form. Coincident with its efficient expression in the heterologous host, the C. tsukubaensis alpha-glucosidase gene contains many of the canonical features of highly expressed S. cerevisiae genes. There is considerable sequence similarity between C. tsukubaensis alpha-glucosidase, the rabbit sucrase-isomaltase complex (proSI) and human lysosomal acid alpha-glucosidase. The cloned DNA fragment from C. tsukubaensis contains a second open reading frame (ORF 2) which has the capacity to encode a polypeptide of 170 amino acids. The function and identity of the polypeptide encoded by ORF 2 is not known.

Structural and functional correlates of sucrase-alpha-dextrinase in intact brush border membranes

The structure and catalytic function of rat intestinal sucrase-alpha-dextrinase (sucrase-isomaltase) were characterized in intact brush border membranes by differential denaturation in 1% SDS at 4, 37, 45, 55, and 100 degrees C, analysis by acrylamide electrophoresis, and subsequent renaturation by transfer to nitrocellulose and in situ analyses of immunoactivity and catalytic activity (immunoblotting and catalytic blotting). Both the sucrase and alpha-dextrinase activities were associated with two mature oligomers, with sucrase predominantly in a 250-260-kDa unit and dextrinase in a 330-350-kDa unit. While sucrase activity declined progressively in response to increasing temperature to 45 degrees C due to loss of active sites, alpha-dextrinase activity increased reciprocally (Vmax +176%). Three principal monomeric products of postinsertional processing comprise the oligomers: alpha, 140 kDa, which carries the sucrase active site; beta, 125 kDa, harboring the dextrinase active site; and gamma, 110 kDa, produced by removal of 185 amino acid residues from the N-terminus of the alpha. Rather than being a simple hybrid dimer, membrane-associated sucrase-alpha-dextrinase appears to consist of two major oligomeric forms having complex structural associations that dramatically affect the availability of the active catalytic sites at the brush border membrane surface.

Posttranslational cleavage of rat intestinal lactase occurs at the luminal side of the brush border membrane

The intestinal sucrase-isomaltase precursor is cleaved at the brush border membrane by luminal proteases. Whether the lactase precursor also is cleaved by luminal proteases is uncertain. Lactase synthesis and processing was studied in 0- and 15-day-old rats after IP administration of [35S]methionine, and changes in precociously cortisone-induced sucrase-isomaltase were used as an internal control. Mucosal lactase and sucrase-isomaltase were separately immunoprecipitated and analyzed by autoradiography after electrophoresis. In both 0- and 15-day-old rats, mucosal lactase appeared as a 200K lactase precursor band at 30 minutes and as 200K and 225K lactase precursor bands at 60 minutes and was cleaved to form a 130K lactase band 120-240 minutes after labeling; sucrase-isomaltase similarly appeared as 210K and 220K bands at 30-60 minutes and was cleaved to form 140K I and 120K S subunits by 240 minutes in day 15 rats. To determine the role of luminal proteases, intestinal segments were isolated in situ and the luminal contents were flushed 30 minutes after labeling. Unflushed segments were used as controls. Only lactase precursor and sucrase-isomaltase precursor were present 240 minutes after labeling in flushed intestinal segments, but lactase precursor and sucrase-isomaltase precursor were cleaved in unflushed segments. Addition of trypsin or elastase into the lumen of flushed segments resulted in partial cleavage of lactase precursor but not of sucrase-isomaltase precursor. Luminal contents collected from the small intestine of day 15 rats 120 and 240 minutes after labeling showed 35S-labeled 130K and 80K polypeptides in lactase immunoprecipitates. It is concluded that intestinal lactase is synthesized as lactase precursor and transported to brush border membrane and cleaved by luminal proteases, and the amino end polypeptide cleaved from lactase precursor is released into the lumen.

Molecular cloning and characterization of a rat intestinal sucrase-isomaltase cDNA. Regulation of sucrase-isomaltase gene expression by sucrose feeding

To investigate the regulation of expression of intestinal sucrase-isomaltase (SI) complex in response to sucrose feeding, we isolated a cDNA (RPSI1) encoding partially the pro-SI of rat intestinal mucosa. The clone consists of 1929 mRNA-derived nucleotides, which covered the region including the C-terminal part of the isomaltase and the N-terminal part of the sucrase in the final SI complex. Nucleotide and amino-acid sequences of RPSI1 were compared with their corresponding regions in rabbit pro-SI. A greater similarity was found in sucrase parts than in isomaltase parts of the two species. Northern blot analysis revealed that the mRNA levels of SI increased rapidly after sucrose force feeding, while those of rats fed a carbohydrate-free diet did not. These changes in mRNA levels correlated with the corresponding enzyme activities. The results demonstrate that the induction of SI activities is directly associated with an increase in SI mRNA levels. Our results also suggest that circadian modulation of SI transcription may occur in basic SI gene expression.

Molecular aspects of disaccharidase deficiencies

We have described the methods used for studying the biosynthesis and the post-translational processing of sucrase-isomaltase (SI), lactase-phlorizin hydrolase (LPH) and maltase-glucoamylase (MGA) in human small intestinal mucosa. Our results are discussed in the context of findings by other researchers. A surprising finding coming out of all these studies is that SI, LPH and MGA are structurally quite different. SI and LPH are both synthesized as large molecular weight precursors which are proteolytically processed to the mature enzymes. In the case of SI, this processing occurs after insertion of the precursor into the brush border membrane and is catalysed by pancreatic proteases; the mature form consists of the two subunits sucrase and isomaltase, the latter containing an N-terminal peptide anchor. Proteolytic processing of the LPH-precursor occurs intracellularly, yielding a mature enzyme in the form of a two active site polypeptide which is anchored via a C-terminal peptide. The role of the large cleaved propolypeptide of LPH is not yet known. MGA is the largest of the three disaccharidases, having a molecular weight of greater than 300 kDa. No proteolytic processing seems to be taking place during biogenesis of MGA in human mucosa, and the mode of attachment to the membrane is unknown at present. The application of the methods described to the investigation of congenital sucrase-isomaltase deficiency (CSID) and lactase restriction in adults is presented and differences between CSID and LPH restriction are discussed.

First purification and characterization of a sucrase-isomaltase from goose kidney microvillous membrane

Goose (Anser anser) kidney microvillus sucrase-isomaltase (EC 3.2.1.48-EC3.2.1.10) was solubilized from isolated microvillus membranes using Emulphogen BC 720 or papain. Detergent-solubilized enzyme (D-SI) was purified 149 +/- 29 times with a yield of 15.7 +/- 2.6% by a two-step procedure which included chromatofocusing. The specific activity was 2.95 +/- 0.34 U/mg protein for sucrase, 1.02 +/- 0.13 for palatinase and 5.63 +/- 0.53 for maltase. D-SI was amphiphilic as indicated by its detergent-binding properties. These properties were not observed for sucrase-isomaltase released from the microvillus membrane by papain. The Mr of the enzyme purified after solubilization by Emulphogen and papain was 543,000 and 380,000, respectively, as determined by gel filtration. The difference in Mr indicates that an Emulphogen micelle is bound to the detergent-solubilized enzyme. In sodium dodecyl sulphate-polyacrylamide gel electrophoresis, sucrase-isomaltase migrated as several polypeptide chains: a major band (Mr 280,000) and at least seven additional minor bands (Mr 220,000-100,000). It is suggested that the major band represents the precursor pro-sucrase-isomaltase and that the lower molecular weight bands are generated by PMSF or aprotinin-resistant proteinases during homogenisation and chromatography of the enzyme. Measured by chromatofocusing, the isoelectric point was found to be pH 4.6. Sucrase-isomaltase accounts for about 20% of total microvillus membrane proteins.

Purification of the rabbit small intestinal sucrase-isomaltase complex: separation from other maltases

The rabbit intestinal sucrase-isomaltase complex has been purified to homogeneity after solubilization with Triton X 100 followed by chromatography on DEAE Sepharose CL 6B and a second solubilization with papain. After hydrophobic chromatography on Octyl Sepharose CL 6B, separation from other contaminating maltases was achieved by gel filtration on Ultrogel ACA 22. The final enzyme was purified 390 fold, with a specific activity of about 10 units per mg protein.

Brush border membrane sucrase-isomaltase, maltase-glucoamylase and trehalase in mammals. Comparative development, effects of glucocorticoids, molecular mechanisms, and phylogenetic implications

1. Trehalase, sucrase-isomaltase and maltase-glucoamylase are three integral glycoproteins of the brush border membranes of the enterocytes. On the basis of a comparative study on alpha-glycosidase activities (sucrase, isomaltase, maltase, glucoamylase and trehalase) associated to these glycoproteins during neonatal development, mammals could be basically divided into three groups. 2. In rodents and rabbit alpha-glycosidase activities are low or undetectable during the suckling period and increase to adult levels during the weaning period. In cat, dog and the primates examined, alpha-glycosidase activities are well or fully developed at birth. 3. In ruminants and pinnipedia alpha-glycosidases are low or absent throughout life. 4. During the suckling period of rat, mouse and rabbit, glucocorticoids trigger a premature and dramatic increase of all alpha-glycosidases. 5. On the contrary, alpha-glycosidases development during the weaning period appears to be independent of glucocorticoids. Neither hypophysectomy nor adrenalectomy prevent the development of alpha-glycosidases; only the rate of increase is reduced. 6. Transplantations of intestinal isografts either in adult or suckling animal, have shown that (1) no systemic factor inhibits the expression of alpha-glycosidase, (2) alpha-glycosidases induction is neither triggered by luminal alimentary substances, nor by hormones, (3) alpha-glycosidase development is controlled by an intrinsic ontogenic program. 7. The use of an antiglucocorticoid failed to inhibit the spontaneous development of alpha-glycosidase activities. 8. The increase of maltase and sucrase activities triggered by glucocorticoids is associated with an increase of the concentration of two glycoproteins in the microvillous membrane: sucrase-isomaltase and maltase-glucoamylase. 9. After administration of glucocorticoids the increase of maltase, sucrase and trehalase is strongly inhibited by actinomycin-D and the increase of sucrase activity is associated with a parallel increase of sucrase-isomaltase mRNA. Transcription is most likely the primary site of control of alpha-glycosidase biosynthesis. 10. In the crypt cells, alpha-glycosidases biosynthesis appears to be triggered by a receptor-mediated glucocorticoid interaction. 11. The enterocytes synthesize more alpha-glycosidase molecules as they travel to the tip of the villi. 12. The simultaneous, biosynthesis of sucrase-isomaltase and maltase-glucoamylase triggered by glucocorticoids, as well as their simultaneous normal development suggest that they may be subjected to related control mechanisms. 13. It is suggested that sucrase-isomaltase and maltase-glucoamylase might have arisen by several cycles of partial gene duplication of an ancestor gene coding for a single site maltase-isomaltase; subsequent mutation would have transformed isomaltase into sucrase or glucoamylase.

Biosynthesis of N-benzoyl-L-tyrosyl-p-aminobenzoic acid hydrolase: disulfide-linked dimers are formed at the site of synthesis in the rough endoplasmic reticulum

N-Benzoyl-L-tyrosyl-p-aminobenzoic acid hydrolase (PPH) is a metalloendopeptidase found in mucosal epithelial cells of the human small intestine. The purification and characterization of this enzyme was described in the preceding paper (E. E. Sterchi et al. (1988) Arch. Biochem. Biophys. 265, 105-118). In this paper, we report on the biosynthesis and posttranslational processing of PPH in organ cultures of human small intestinal mucosa. Continuous labeling for 6 h with L-[35S]methionine, immunoprecipitation with monoclonal antibody 3/716/36, and analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a polypeptide with Mr 100,000. This molecule was highly glycosylated as treatment with endo-beta-N-acetylglucosaminidase F resulted in a reduction to Mr 70,000. This was also the size of the species isolated after culture in the presence of tunicamycin, an inhibitor of N-linked glycosylation. Pulse-chase labeling showed that the first detectable form of PPH had a Mr 90,000 which corresponded to the high-mannose precursor as assessed by its sensitivity to endo-beta-N-acetylglucosaminidase H. Within 15 min of chase and prior to complex glycosylation, dimerization due to the formation of interchain disulfide bonds occurred (Mr 180,000). Dimerization thus took place within the rough endoplasmic reticulum and might play an important role in the transport through to the cell surface. After 2 h of chase, PPH started to appear in the culture medium, indicating that the enzyme was secreted from the cells, a finding not observed with other microvillus membrane hydrolases.

Regional assignment of the gene coding for human sucrase-isomaltase (SI) to chromosome 3q25-26

The gene coding for sucrase-isomaltase (SI) has recently been mapped to chromosome 3 using a cDNA probe to analyse DNA from somatic cell hybrids (Green et al. 1987). We have now used this same cDNA probe to obtain a regional localization of this gene. In situ hybridization to normal metaphase chromosomes and to chromosomes from individuals with balanced translocations suggests a regional assignment to chromosome 3q25-26.

A study of the molecular pathology of sucrase-isomaltase deficiency. A defect in the intracellular processing of the enzyme

The intestinal brush-border enzyme sucrase-isomaltase splits sucrose into its component monosaccharides, glucose and fructose. A deficiency of the enzyme leads to sucrose intolerance. We studied the synthesis and intracellular processing of sucrase-isomaltase, using human intestinal explants in organ culture. Pulse-chase experiments with [35S]methionine followed by immunoprecipitation, sodium dodecyl sulfate-polyacrylamide-gel electrophoresis, and fluorography of labeled sucrase-isomaltase demonstrated that the molecule was initially recognized as a protein with a relative molecular weight (Mr) of 205,000. This was apparently converted to a species of 225,000 Mr within two hours. We studied the glycosylation of the protein using endo-beta-N-acetylglucosaminidase H and peptide-N4-(N-acetyl-beta-glucosaminyl)-asparagine amidase digestion of oligosaccharide side chains of the two forms of sucrase-isomaltase. The results showed that the early-appearing 205-kd (kilodalton) molecule contained high-mannose asparagine-linked oligosaccharides, and that the later-appearing, 225-kd molecule contained highly processed (mature) carbohydrate chains. Studies in a patient with primary sucrase-isomaltase deficiency demonstrated normal translation and high-mannose glycosylation of the precursor but a failure in further processing of the oligosaccharides, with subsequent intracellular degradation of the glycoprotein and undetectable enzymatic activity of intestinal sucrase. Abnormal intracellular processing of the enzyme was the probable mechanism of enzyme deficiency in this patient.

The mode of anchoring and precursor forms of sucrase-isomaltase and maltase-glucoamylase in chicken intestinal brush-border membrane. Phylogenetic implications

Chicken intestinal sucrase-isomaltase and maltase-glucoamylase have been isolated in their intact form by detergent solubilization and characterized as to their subunit composition and mode of anchoring in the brush-border membrane. Both are heterodimeric enzyme complexes composed of two subunits each of approximately 140 and 130 kDa. Contrary to the mammalian sucrase-isomaltase, chicken isomaltase was identified as the smaller of the two subunits. As was shown by hydrophobic labeling, only one of the two subunits in each heterodimer is anchored in the bilayer, the smaller 130 kDa isomaltase subunit of the sucrase-isomaltase complex, and the larger 140 kDa subunit of the maltase-glucoamylase complex. Both preparations contain a high-molecular weight polypeptide of approximately 250 kDa which in the case of sucrase-isomaltase could be identified by peptide mapping as a single-chain precursor not (yet) proteolytically processed to the final heterodimer. These first data on the mode of membrane anchoring of non-mammalian glycosidases indicate that they are synthesized, inserted into the membrane, and processed in ways similar to the mammalian enzymes. The fundamental unity between avian and mammalian sucrase-isomaltases suggests that the partial gene duplication of an ancestral isomaltase gene and the subsequent mutation of one of the active sites resulting in pro-sucrase-isomaltase has occurred prior to the separation of mammals from reptiles, i.e. more than 300 million years ago.

Isolation of a cDNA probe for a human jejunal brush-border hydrolase, sucrase-isomaltase, and assignment of the gene locus to chromosome 3

We report the nucleotide sequence and derived amino acid sequence of a cDNA clone encoding most of the N-terminal, isomaltase region of human sucrase-isomaltase (SI). A plasmid containing this cDNA, pS12, identifies a 6-kb mRNA found in human jejunum and the human colon carcinoma cell line Caco-2. This human SI cDNA shows extensive overall homology with recently published rabbit SI cDNA. Using pS12 to probe DNA from a panel of somatic cell hybrids, we have assigned the gene encoding human SI to chromosome 3.

The sucrase-isomaltase complex: primary structure, membrane-orientation, and evolution of a stalked, intrinsic brush border protein

The complete primary structure (1827 amino acids) of rabbit intestinal pro-sucrase-isomaltase (pro-SI) was deduced from the sequence of a nearly full-length cDNA. Pro-SI is anchored in the membrane by a single 20 amino acid segment spanning the bilayer only once. The amino-terminal, cytoplasmic domain consists of 12 amino acids and is not preceded by a cleaved leader sequence. This suggests a dual role for the membrane-spanning segment as an uncleaved signal for membrane insertion. This is followed by a 22 residue serine/threonine-rich, probably glycosylated, stretch, presumably forming the stalk on which the globular, catalytic domains are directed into the intestinal lumen. Following this is a high degree of homology between the isomaltase and sucrase portions (41% amino acid identity), indicating that pro-SI evolved by partial gene duplication.

Presence of sucrase in the yolk sac of the chick

The presence of sucrase in the yolk sac of the chick was studied biochemically and immunologically. The sucrase was partially purified from the yolk sac of hatched chicks and was compared with the sucrase purified from the small intestine. Immunodiffusion with antiserum against intestinal sucrase and characterization of the activity revealed that the two enzymes were almost identical. However, the size of the yolk sac sucrase was found to be slightly smaller than that of the intestinal enzyme by chromatography on Sephadex G-200 and polyacrylamide gel electrophoresis. Immunocytochemical studies showed that the sucrase was located on the free surface of yolk sac endodermal cells, but the sucrase may also be present in the cytoplasm.

In vitro translation of intestinal sucrase-isomaltase and glucoamylase

It is has been proposed that both sucrase-isomaltase and glucoamylase are initially synthesized as large single-chain precursors which are then processed to heterodimers. We have tested this hypothesis by in vitro translation of their mRNAs. The primary translation product of sucrase-isomaltase mRNA was a single polypeptide of Mr = 190,000. Similar experiments using antiserum against glucoamylase revealed a single polypeptide of Mr = 145,000. These results are consistent with the single chain precursor hypothesis for sucrase-isomaltase. However, the glucoamylase product (145 kDa) is too small to be processed to a heterodimer of Mr = 230,000. Moreover, the mature subunits (Mr = 135,000 and 125,000) probably are derived from the 145 kDa precursor by proteolytic trimming and must include and share most of the precursor protein.